1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and in particular, to a sheet feeding apparatus configured to feed a stacked sheet to a body of an image forming apparatus one by one from the top.
2. Description of the Related Art
Nowadays, as an image forming apparatus such as a copying machine, a printer, and a facsimile machine, an image forming apparatus including a sheet feeding apparatus feeding a sheet, on which an image is to be formed, to an image forming portion, is widely used. Generally, in a sheet feeding apparatus, a sheet is stacked on a tray capable of being lifted up to and down from a sheet feeding cassette, and the sheet stacked on the tray are fed by a pickup roller provided in an apparatus body.
In the sheet feeding apparatus, for cost reduction, the pickup roller is driven by a motor capable of rotating clockwise and counterclockwise, and simultaneously, the tray is lifted and lowered. For example, a sheet feeding operation is performed by driving the pickup roller by clockwise rotation of the motor, and a lift-up operation is performed by lifting the tray by counterclockwise rotation of the motor. That is, both the sheet feeding operation and the lift-up operation are performed by using one motor to switch motor rotation between a clockwise direction and a counterclockwise direction, thereby reducing the cost of the apparatus.
Also, as a sheet feeding method, there is a method that abuts a liftable and lowerable pickup roller on a sheet by lowering the pickup roller when the sheet is fed, and lifts the pickup roller after the fed sheet reaches a separating portion or a conveying portion of a downstream side. In this manner, the uppermost sheet can be certainly fed separately by lifting and lowering the pickup roller whenever the sheet is fed.
In this method, a sensor that detects a position of the pickup roller is provided. In feeding the sheet, the sensor detects a position when the pickup roller has been lowered. At the time of sheet feeding, the pickup roller is lowered by rotating the motor clockwise, and the height of the top surface of the uppermost sheet (hereinafter, referred to as a sheet surface height) on the tray is determined based on the detection from the sensor after a lapse of a predetermined time. When it is determined based on the detection of the sensor that the height is low, the uppermost sheet moves to a height of a feedable range by performing a lift-up operation by rotating the motor counterclockwise before a next sheet feeding operation (see U.S. Pat. No. 5,988,628).
However, nowadays, improvement in productivity (number of image-formed sheet per unit time) is required from market demands. In order to realize this, it is necessary to increase a process speed or narrow a sheet interval (interval between before and after the successively conveyed sheet). However, if the process speed is increased or the sheet interval is narrowed, a sheet feeding interval (sheet discharging interval) from a sheet feeding cassette is shortened.
As such, in the control that lowers the pickup roller when the sheet feeding interval is shortened, and performs the lift-up operation by determining the lift-up operation after a lapse of a predetermined time, it is difficult to match a timing of a sheet feeding operation with a feeding timing of a next sheet after the fed sheet. That is, there is a risk that feed failure will occur because the lift-up operation is not ended at the feeding timing.
FIG. 14 is a flowchart of a sheet feeding sequence of the sheet feeding apparatus according to the related art. In the sheet feeding apparatus according to the related art, a sheet feeding operation is started, and a sheet is fed by rotating a pickup roller while being pressed on a sheet (Step 11). Next, the pickup roller is lifted (Step 12). Next, when a rear end of the fed sheet passes through a separating portion, the pickup roller is lowered at substantially the same time (Step 13). Next, when 50 ms has elapsed, it is determined whether a sheet surface height is a lower limit, based on detection of a detection sensor (Step 14). When the sheet surface height is not the lower limit (NO in Step 14), a next sheet is fed by rotating the pickup roller (Step 11). Also, the lower limit of the sheet surface height refers to a case where a height of a top surface of an uppermost sheet stacked on a tray is a lower limit of a sheet feedable range. When it is determined that the sheet surface height is the lower limit, the height of the top surface of the sheet is lifted by lifting the tray.
Also, when the controller determines that the sheet surface is low, that is, the sheet surface height is the lower limit, based on the detection of the detection sensor (YES in Step 14), the controller performs the lift-up operation until the sheet surface height becomes the sheet feedable range, based on the detection of the detection sensor (Step 15).
Also, as a timing of detecting the sheet surface height, the detection of the sheet surface height is performed after a predetermined time (for example, 50 ms) from a pickup roller lowering start signal, from an electrical delay and a mechanical operation time of a solenoid for lifting the pickup roller. Also, in order to secure the stable sheet feeding operation, a lift-up time of a predetermined time (for example, minimum 100 ms) is required as a time for performing the lift-up operation. The reason why the 100-ms lift-up operation is required is to solve a transmission loss caused by backlash of a gear at the time of switching the clockwise/counterclockwise rotation of the motor.
Also, in this sequence, after the lift-up operation, a next sheet feeding operation is performed after a lapse of a stabilization time of the motor. This is because when the sheet feeding and lift-up operation by the pickup roller is performed by the switching of the clockwise/counterclockwise rotation of the motor, the motor needs to be stopped once at the time of switching the rotating direction. Also, there is a risk that the motor will step out when the rotating operation is started without stabilization time. For this reason, the stabilization time of, for example, 100 ms, is provided after the stop.
As such, in the existing sequence, if minimum 250 ms has not elapsed after the sheet is fed, the next sheet feeding operation cannot be performed. Therefore, it is impossible to sufficiently cope with the demand for the improvement of productivity. That is, in the conventional sequence, the sheet feeding interval cannot be set to be short, it is impossible to sufficiently cope with the demand for the improvement of productivity.
Therefore, the present invention has been made in view of such circumstances, and is directed to provide a sheet feeding apparatus and an image forming apparatus, which can set a sheet feeding interval to be short.